The invention relates generally to the field of aerogels, and more particularly to a method of using aerogels to form net-shaped materials and perform net-shaping.
Aerogels are unique solids with up to 99% porosity. Such large porosities confer a number of useful properties on aerogels, including high surface area, low refractive index, low dielectric constant, low thermal-loss coefficient, and low sound velocity. To date, however, the potential of aerogels has not generally been realized in these applications because conventional supercritical aerogel processing is energy intensive and often dangerous. Silica aerogels, with thermal conductivities as low as 0.02 W/mK, have potential utility in superinsulation systems.
Traditionally, aerogels are made by processes whereby the liquid contained within the continuous network of pores of a gelatinous solid is replaced by air. Typically, this is achieved by supercritical solvent extraction, i.e., by placing the gel in an autoclave where the temperature and pressure is increased above the critical point of the liquid phase. This process was initially proposed by Kistler (Kistler, U.S. Pat. No. 2,249,767) to avoid the shrinkage and cracking of porous materials (water filled) due to capillary forces generated during simple evaporative drying. Improvements to Kistler's process were developed. Notably, Nicolaon and Teichner (Nicolaon et al., U.S. Pat. No. 3,672,833) supercritically dried silica gels under conditions exceeding the critical point (240.degree. C., 78.5 atm) of the methanol solvent contained within the pores of a gel. Tewari and Hunt (U.S. Pat. No. 4,610,863) developed a process whereby the initial pore fluid (alcohol) is exchanged for carbon dioxide (31.degree. C., 72.9 atm), thus reducing the temperature required for processing and enhancing process safety by the elimination of flammable solvents at high pressure.
In another advance in aerogel processing, Deshpande et al. (Deshpande et al., U.S. Pat. No. 5,565,142; incorporated herein by reference) describe a means for surface modification of the wet precursor gel to change the contact angle of the fluid meniscus in the pores during drying to avoid shrinkage of the gel. In another advance in aerogel processing, Brinker et al. (Brinker et al., U.S. Pat. No. 5,948,482; incorporated herein by reference) describe a low temperature/pressure (LTP) process to form thin films, eliminating the need for supercritical processing by chemical derivatization of the wet gel surface, followed by simple drying under ambient temperature and pressure conditions. The chemical surface treatment causes the drying shrinkage of the thin films to be reversible: during drying the gel thin film shrinks, then re-expands to recreate the porosity and volume of the wet gel state.
Because aerogels are made by sol-gel processing, their microstructure can be tailored to optimize properties desired for specific applications. Various precursors, including metal alkoxides, colloidal suspensions, and a combination of both under several mechanisms of gelation may be used to synthesize gels. Aerogels can also be made from wet precursor gels that contain both inorganic and organic components or from organic gels. For the composite gels, the organic and inorganic phases can be mixed on different length scales such that the organic component resides solely on the internal pore surface, is incorporated into the spanning gel structure, or forms a separate gel structure from the inorganic phase.
Some applications, such as insulation with a cavity of a complex shape, require materials that can form to the shape of the mold or cavity and provide desired properties. In some applications, foams are suitable materials for such uses. Aerogels, with their high porosity and low thermal conductivities, could be used for such applications. However, aerogels have not been used because the inherent limitations of conventional supercritical routes to aerogels, such as high pressure autoclave processing and difficult processing of large or complex shapes, have contributed to high processing costs and thus have severely restricted successful commercial development of aerogel processes for these type of applications.
Virtually all existing aerogel processes for the fabrication of bulk aerogel materials, including supercritical processing and low temperature/pressure (LTP) processing, depend upon expensive molding and machining techniques to fabricate parts with controlled geometries. For example, to obtain shaped articles using conventional processes, one must cast the sol into a suitable mold and process the gel in a pressure chamber (such as an autoclave) that is large enough to contain the molded shape. These processes are suitable, albeit expensive, for small simple shapes; however, they are unsuitable for complex shapes or for applications that demand cost-effective manufacturing. For most applications, current techniques impose severe restrictions to facile manufacturing; for example, molding technology requires precision mold machining, casting technology, mold material compatibility, custom mold design, effective mold release agents, and controlled part shrinkage while cost-effective machining of aerogels is difficult due to their fragility. Some of these limitations can be overcome by the use of granular aerogel materials, which are more manufacturable but are unsuitable for many applications requiring bulk shapes because of the large voids between granules.
Useful would be a method of preparing net-shape aerogel materials for a range of applications that avoids the disadvantages and limitations inherent in conventional supercritical processing and exploits and improves upon the advantages of recent low temperature/pressure (LTP) processing.